This last
week was the final Dipterists Forum field meeting of the year (10-17 October):
a combined visit to Dorset and Hampshire. These meetings focus on craneflies
and fungus gnats, with the additional hope of Platypezids and Heliomyzids.
The first 4
days were based at Dorchester and concentrated on the Isle of Purbeck. The main
localities visited were Studland NNR and Arne Heath. The second half of the
week we transferred to Ringwood and focussed on the New Forest.
As in many
recent years, we had nice sunny weather but this year was a little cooler. The
temperature meant that we did not get started until 11 am each day, and by 3.30
there was very little moving. It also meant that we generally only managed to visit
3 localities each day.
The numbers
of flies seen were remarkably low and it is difficult to be sure why. It was
certainly cool, and although there was sunshine it was difficult to find warm
places where flies gather. Perhaps numbers were genuinely low? If so, why might
this have been? Fly larvae need damp
conditions: periods of drought and high temperatures can kill delicate larvae.
We can be fairly sure that long periods of drought have played a part in
declining fly numbers over the past 40 years. Many wetland species took a long
time to recover after the droughts in the mid-1970s and were hit again in the
early 1990s and early 2000s.
The low
catch of craneflies seems to reflect an ongoing trend in which the numbers and
diversity are declining year-on-year. My own hauls of fungus gnats were equally
pathetic – I rarely managed to assemble a sample of more than 20 specimens in
an hour’s sweeping and we rarely exceeded a site list of 20 species all week.
When you bear in mind that we had a trip to Sussex a few years back, in which
we recorded the better part of 200 species and assembled several site lists
exceeding 90 species, this year has not been a good one despite visiting one of
the most important saproxylic sites in the country. Numbers of Platypezids were
a little better, with seven or eight species recorded. With these I did a
little better than in previous years but still only managed two species and a
handful of specimens.
Data for the
hoverfly Rhingia campestris show how
this species is substantially knocked out by periods of drought, with the
summer generation failing to materialise after intense hot spells in July. One
possible explanation for the lack of flies this autumn is therefore that the
intense hot spell in July was responsible for the death of many fly larvae.
That cannot be the whole story, however, because we saw good numbers of
Helemyzidae (not many species) at several sites in The Forest. Another
possibility is that gnat numbers were depressed because there were relatively
few fungal fruiting bodies. But, then again, there were patches of honey
fungus, Armillaria sp., but very few
gnats in attendance. So, perhaps it was just too cold?
When I
sought permission to visit the New Forest I was aware that there was concern
about my mention of fungi – until I clarified that we were interested in
fungus-feeding flies. There are big problems with commercial mushrooming, with
many areas stripped of all fruiting bodies. So, unsurprisingly, any mention of
fungi will inevitably raise concern. In the event, our visit was not a problem
in this respect and we hope that the data produced will be helpful. In many
places fungi were notable by their absence, but then again we did not see large
numbers of fungi anywhere on our travels this autumn (the best meeting I can
remember was in Radnorshire about ten years ago).
The reasons
for low fly numbers must remain unresolved. There are a number of possible
factors but none can completely explain our experience, which goes to show how
much more there is to learn about the factors that influence invertebrate
abundance.
Despite low
numbers of flies there were a few high points. One of the most noteworthy
records was that of Scathophaga
scybillaria – a highly localised species of Scathophagidae with
long dusky wings that might be mistaken for a big S. stercoraria. Prior to last year I had never seen this species
but we found it fairly abundantly on bogs on the Lleyn Peninsula during the
summer field meeting based at Bangor. It was great to see further examples in
the New Forest (caught by Alan Stubbs). Malcolm Smart caught a specimen of the
Keroplatid Keroplates testacea – a
very large fungus gnat that at one time was very noteworthy but in recent years
has proven to be far commoner and widespread than previously thought. Alan
Stubbs also recorded the Mycetophilid Greenomyia
mongolica which is a recent addition to the GB list. One further interesting observation was that a few of the ponies were heavily covered in eggs of the bot fly Gasterophilus equinus - illustrated.
Gasterophilus equinus eggs attached to hairs on the upper foreleg of a New Forest pony. The ponies lick these off and ingest them where the larvae develop in their gut before passing out in the dung. |
Observations on the New Forest
We visited a
wide range of localities around the New Forest, giving a helpful overview of
its composition and management. On the few previous occasions that I have
visited The Forest, I have been struck by the level of grazing pressure. Much
of The Forest is so heavily grazed that the turf would make a fine bowling
green (making allowances for pony dung!). Even so, I don’t think I have ever
previously seen Devil’s Bit Scabious 3 cm high and in flower! This management
might maintain the plant composition, but from a Dipterist’s perspective it
makes much of The Forest very uninteresting. Nice flushes with 2cm high turf is
of precious little interest, so huge areas of The Forest have been effectively
sterilised from that perspective.
In several
places we noted the same grazing pressure within inclosures and wooded areas.
Heather and bilberry a few centimetres high do not provide cover for flies and,
if they do survive, they are extremely difficult to sample. It was darned hard
work securing a reasonable sample in localities that are reputedly rich in
wildlife. In the end I got to the point where I ran out of enthusiasm for
sweeping over-grazed habitat and ceased sampling.
Pony lawn as Shepherds Gutter. This turf has some very nice wet flushed areas with Devil's-bit scabious but spot the flowers. |
Miniature Devil's-bit scabious! |
A few years
ago, Dipterists Forum visited The Forest in May and similar concerns about
grazing pressure were expressed. The issue generated a certain amount of debate
but we found ourselves out on a limb, with other specialist groups arguing that
there had been no change in recent decades and that all the speciality species
were still present. I find that very hard to believe. Perhaps grazing pressure
has remained constant; in which case I suspect that the species of interest
that may once have occurred have long-since disappeared, or are confined to
very small parts of The Forest. I don’t think that this factor can be
attributed to the loss of Eristalis
cryptarum but we just don’t know what else once occurred and has since been
lost!
It is worth
bearing in mind, however, that assessing the condition of the New Forest is highly
subjective. Whilst at Denny Wood, Keith Alexander commented that he thought it
was in excellent condition as an example of wood pasture. That might be true
for Coleoptera associated with ancient trees, but even then there are indications
of a problem with grazing.
Veteran beech and oak at Denny wood. |
Beech in various states of development at Rufus Stone |
At the
moment there are a good number of veteran beech trees within The Forest but I was surprised, even shocked, to note how
many had been blown over in the last two to three years. I suspect this period
will have resulted in quite serious depletion of veteran beeches. The loss of a
significant component of this one tree species in a short time-frame is arguably
very serious, especially when one bears in mind that it is very important for a
wide range of saproxylic Diptera. I therefore suggest that the debate about
grazing pressure is not over. There is a need for action to address the rapid
loss of beech, and to think about the wider implications of current grazing
pressure.
These beech
trees are often tall and straight-stemmed, indicating that they grew in high
forest situations. They are much the same age and must have germinated within a
matter of years of each other. Surely, if large numbers of beech trees survived
around the same time that is indicative that the prevailing conditions suited
their survival? Currently, one sees good numbers of young (1-2 year old)
saplings but no older ones: they are grazed away pretty rapidly. So, I suggest
that there must have been periods of lower grazing pressure that have allowed
recruitment of new trees. In which case, grazing pressure has not been
constant. Looking at the age structure in many of the wooded areas, one gets
the impression that there has been little recruitment for between 30 and 50
years (possibly less severe at Mark Ash and really quite good at Pondhead
Inclosure), which means that at some time in the future there will be a
significant gap in the availability of veteran beeches and the saproxylic
resources they provide.
Doubtless,
some of the Inclosures and wetter bogs remain rich for Diptera and other
insects, but The Forest is a huge area. If the majority is grazed to within an
inch of its life, the ratio of rich wildlife to surface area rapidly diminishes
to levels that one sees in a wider landscape context. Instead of a patchwork of
nice habitat amongst an agricultural setting, there is a patchwork of
interesting locations amongst a sea of short turf. Neither is particularly
ideal and the same issues of island biogeography are likely to apply. Isolated
populations are more at risk than populations that are connected by good
habitat. Unfortunately, the data on distribution and composition of Diptera in
The Forest are fragmentary and lack continuity of effort.
It is
therefore neigh-on impossible to place the current situation into a historical
context. Yes, the honeypot sites remain, and yes the charismatic species are
still regularly seen (although data available to the HRS suggests that this is
not the case because we rarely get records from The Forest). But, does this
really mean that The Forest is actually being managed in a way that maintains
its biological diversity? From a dipterological perspective, I suspect that most emphasis is placed on charismatic woodland species, whilst those
animals that are associated with open flushed systems are largely overlooked or
ignored (with the honourable exception of Steve Falk’s interest in The Forest).
In theory
one might effect change by lifting grazing pressure; after all, if the soils
are undamaged (in terms of herbicide, pesticide and nutrient application) then
there is a far better chance of restoring the broad range of animals that might
previously have occurred. It would be very informative to see trial exclosures
from some of the heavily grazed flush systems. Will this ever happen? I doubt
that this is likely or perhaps even possible given limitations on the ability
of managers to exclude grazing animals.
It seems to
me that there is an urgent need to draw together all of the historic data on
the invertebrate fauna of The Forest in order to establish what is really known
about species that are more cryptic or do not attract the attention of those
general natural historians that visit. I understand that this has been done for
the Coleoptera but apparently not for the Diptera. It is a huge task as there
must be many records from the late 19th and early 20th
Centuries lodged as museum specimens.
If it is
unrewarding to visit anywhere but known honeypot sites, then there is little
chance that the grazed lawns will receive any attention and the myth of The
Forest will be maintained by reports from the historic ‘hot spots’. For me,
that is not attractive – I favour visits to less well known areas in order to
improve overall knowledge of our fauna. So, return visits to The Forest are not
frightfully appealing. That will doubtless be the case for many other
Dipterists, so the problem of data weaknesses will be compounded in years to
come.
A reflection on field meetings
So, as
another field season draws to a close it is interesting to reflect on the
successes. We have held three field meetings (Norfolk in May, Nottingham in
July and the Isle of Purbeck and the New Forest in October). The numbers
attending the Norfolk meeting were very encouraging, with nearly 30 Dipterists
attending for all or part of the weekend. Nottingham attracted far fewer
participants than previous summer field meetings but was successful because we
managed to visit a range of sites in an area that has not been heavily worked
for some of the less popular families. And, this autumn? Well of course it was
a success – we had a similar-sized group to previous years (8) and gathered important
data for a range of sites. Perhaps what is even more important is our having
maintained the tradition of autumn field meetings and extended the run to more
than 40 consecutive years.
Dipterists Forum members at Studland NNR. L-R: Chris Spilling, Peter Chandler, Malcolm Smart, Andrew Halstead, Alan Stubbs and Tony Davis |
Dipterists Forum members at Pondhead Inclosure. L-R: Slan Stubbs (photographing an uncooperative spider), Tony Davis, Malcolm Smart, Andrew Halstead, Keith Alexander and Peter Chandler. |
This was the
last year that I will run a summer field meeting. I have been running them
since 2005 and have run out of steam. We have nobody that is willing to take
the job on, although there will be a meeting in 2016 thanks to a combined
effort by Howard Bentley and
Amanda Morgan. I shall probably continue to organise spring and autumn
meetings but I am finding it increasingly difficult to maintain impetus. As yet
I have not fixed on venues for these meetings, although it is likely that one
will be in Scotland in late August/early September. The spring meeting may well
be in Somerset as I think I can find suitable accommodation there.
Unfortunately, as accommodation costs spiral, it is increasingly difficult to
find affordable venues and as one of those with very limited income I am
acutely aware of the cost implications for members who wish to participate!